Is there a design benchmark for lighting?
The lighting design process should take into account:
- Illuminance levelsIlluminance (or light) levels indicate the amount of light required to perform a task in a safe and efficient manner.
- Illuminance uniformityTo create a more comfortable and productive environment, it is recommended that task areas are illuminated in as uniform a way as possible. A qualitative expression of illuminance uniformity is the uniformity ratio, which is defined as the ratio of minimum illuminance levels to average levels. Typically, a ratio of 0.7 is required for creating a comfortable visual environment and avoiding glare. Illuminance uniformity is often achieved by installing light sources spaced in a regular grid over the area.
- Luminance distributionThis refers to the variations in brightness within the field of view, between the task area and its close and distant visual surroundings.
- Colour renderingThe Colour Rendering Index (CRI) is a measure of the ability of a light source to accurately portray the colours of the objects and surfaces it illuminates. The higher the CRI value, the more accurately the colours will be reproduced. Often, CRI values are referred to as Ra (average rendering) values.
- Colour temperatureThe term describes the overall colour of a light source. Different types of natural and artificial light sources produce different degrees of ‘warmth’ (towards yellow) or ‘coolness’ (towards blue). For example, the diffused light from the sky on a cloudy day is perceived by the human eye as being white in colour, whereas the direct light from the sun on a day with clear sky is perceived as yellow or orange in colour. Artificial light sources are categorised on the basis of their colour temperature in a similar way.
- Appropriate use of interior light surfaces, e.g. walls or ceiling
- Additional aspects such as glare and flickerGlare is defined as the sensation produced by intense light in the field of view (which lowers illuminance uniformity). Flickering refers to rapid and repeated changes in the brightness of light, that can cause distraction and possibly physiological effects such as eye strain or headaches.
In the UK, the Society of Light and Lighting (SLL) ‘Code for Lighting’ is considered one of the most comprehensive standards for lighting design. The Code for Lighting presents a holistic approach to lighting design, including energy efficiency, visual function, architectural integration, visual amenity, and both capital as well as operation and maintenance costs.
- SLLCL SLL Code for Lighting: www.cibse.org
- Lighting Digest, A list of British Standards related to the lighting industry: www.lightingdigest.co.uk
- CIBSE, Lighting for people, energy efficiency and architecture – an overview of lighting requirements and design (1999): www.cibse.org
- Waveform Lighting, Guide to the Color rendering index: www.waveformlighting.com